Use nio4r 2.5.2 explicitly
Use nio4r 2.5.2 explicitly

This is because nio4r 2.5.3 requests to use devkit forcibly on Windows platform.
On omnibus Ruby, we cannot handle it.

Signed-off-by: Hiroshi Hatake <a476a6e5b5893650242fbf0b2961500a9bd9032e@clear-code.com>

treasure-data/omnibus-td-agent,treasure-data/omnibus-td-agent,treasure-data/omnibus-td-agent